(1) Field of the Invention
This invention relates to a semiconductor device and, more particularly, to a semiconductor device which operates at an external power supply voltage and an internal power supply voltage being different from the external power supply voltage in voltage.
(2) Description of the Related Art
To increase processing speed and reduce power consumption, in recent years circuits on many semiconductor devices, such as central processing units (CPUs), have been operated at voltages (1.8-3.3V, for example) lower than 5V which is a power supply voltage for conventional interfaces, such as transistorxe2x80x94transistor logic (TTL) circuits.
In not a few systems, however, conventional interfaces, such as TTL circuits, are used as external circuits to reduce the cost. Therefore, in these systems, an internal circuit, being a CPU, is operated at a low voltage, but a power supply voltage of 5V is used for external circuits.
As a result, usually a power supply voltage of 5V is used for an entire system and a step-down circuit is included in the system. This step-down circuit generates a lower voltage and supplies it to semiconductor devices, such as a CPU, as power supply voltage.
FIG. 13 is a view showing an example of such conventional semiconductor devices.
As shown in FIG. 13, a conventional semiconductor device 1 comprises a step-down circuit 2, internal circuit 3, and level converter 4.
The step-down circuit 2 drops power supply voltage Vcc2 (5V, for example) supplied from the outside to generate Vcc1 (3.3V, for example) and supplies it to the internal circuit 3 and level converter 4.
The internal circuit 3 is, for example, a CPU and performs various operations with Vcc1 supplied from the step-down circuit 2 as power supply voltage.
The level converter 4 converts an internal signal (signal at the Vcc1 level) output from the internal circuit 3 into an external-level signal (signal at the Vcc2 level) and outputs it to the outside of the semiconductor device 1 as an output signal.
FIG. 14 is a timing chart for describing the operation of the conventional semiconductor device shown in FIG. 13. As shown in FIG. 14(A), when the supply of the power supply voltage Vcc2 to the semiconductor device 1 is begun, it will be supplied to the step-down circuit 2 and level converter 4.
The step-down circuit 2 drops Vcc2 to Vcc1. As shown in FIG. 14(B), it will take a certain period of time for voltage to stabilize. The internal circuit 3 is in a transient state until Vcc1 stabilizes, and an internal signal output from the internal circuit 3 (see FIG. 14(C)) is indefinite. Vcc1 is also supplied to the level converter 4. The level converter 4 therefore is in a transient state until Vcc1 stabilizes.
Vcc2 is also supplied to the level converter 4. Vcc2 rises faster than Vcc1, so a signal output from the internal circuit 3 and part of a signal generated in the level converter 4 will be output to the outside as an output signal.
As a result, as shown in FIG. 14(D), indefinite output generated by the internal circuit 3 etc. will be output to the outside as an output signal after Vcc1 beginning to rise and before Vcc1 stabilizing. Malfunction therefore can occur in an external circuit connected to the semiconductor device 1.
In order to address such a problem, the present invention was made. In other words, an object of the present invention is to provide a semiconductor device which can prevent malfunction at the time of power being applied.
In order to achieve the above object, a semiconductor device which operates at an external power supply voltage and an internal power supply voltage being different from the external power supply voltage in voltage is provided. This semiconductor device comprises a processing circuit which operates at the internal power supply voltage for performing a predetermined process, an output circuit for outputting the result of processing by the processing circuit, a control circuit which operates at the external power supply voltage and controls so that output from the output circuit is kept in a predetermined state for a predetermined period of time after the beginning of the supply of the external power supply voltage, and a supply circuit for supplying the external power supply voltage to the control circuit.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.